1. Field of the Invention
The present invention relates to a method for forming a bent pipe and an apparatus for the same, particularly to a method and an apparatus for bending an end portion of the pipe.
2. Discussion of the Related Art
A known technique for bending a pipe is disclosed in Japanese patent publication No. 6-42967, a main part of which is shown in FIG. 5. As shown in FIG. 5, an expansion plug 52 is pivotally connected to a tip of a mandrel 51 which is inserted in a pipe 53 and guides the pipe 53 from inside. A diameter of the expansion plug 52 is larger than an inner diameter of the pipe 53. The pipe 53 being advanced along the direction shown by arrow A in FIG. 5 is expanded as it arrives at the expansion plug 52. A bending force (shown at an arrow B in FIG. 5) is applied to the expanded pipe 53 by a bending member 54. Accordingly, the pipe 53 is bent and expanded concurrently.
According to this method of bending the pipe, the expansion plug 52 is biased toward the bending direction by the bending force applied by the bending member 54. So a circumferential inclination stress is generated in the pipe 53. The inclination stress generated at an inner side wall C of the pipe 53 is large, and the inclination stress generated at an outer side wall D of the pipe 53 is small. This stress generates another bending force. Thus, two bending forces are added to the pipe 53: the bending force by the bending member 54 and the bending force due to the inclination stress. So it is possible to bend the pipe with a small force.
Further, due to the bending force generated by the inclination stress, the thickness of an inner side wall C of the pipe 53 becomes smaller. On the other hand, by the bending force generated by the bending member 54, the thickness of the outer side wall D of the pipe 53 becomes even thinner. Therefore, by bending the pipe using two bending forces, the thickness of the whole wall of the pipe can be uniform along the circumferential direction.
In the operation of the prior art described above, before forming the bent pipe the bending member 54 is located below the mandrel 51 (as shown at 54' in FIG. 5). If the bending member is located at the position shown at 54 (not 54') before starting to bend the pipe, the pipe may buckle. Upon starting, the pipe 53 advances in the direction shown by the arrow A in FIG. 5 and is expanded by the expansion plug 52.
When an end portion of the expanded pipe 53 reaches the bending member 54', the bending member 54' moves to the location shown at 54 in FIG. 5. By further advancing the pipe 53 with the bending member located at position 54, the pipe is bent with a predetermined radius.
An end portion of the pipe passed through the expansion plug 52 before the bending member arrives at position 54 is not bent with a predetermined radius, but with a radius larger than the predetermined radius. This is because this portion of the pipe does not receive sufficient bending force by inclination stress generated when the pipe passes through the expansion plug 52.
Another prior art is shown in Japanese patent publication No. 5-12047, a main part of which is shown in FIG. 6. As shown in FIG. 6, a pipe 55 is inserted in a guide cylinder 56 which guides the pipe 55. A bending die 57 is placed downstream of the guide cylinder 56. The guide cylinder 56 externally guides the pipe 55. The bending die 57 deviates from the central axis line of the guide cylinder 56 to produce an offset u. Accordingly, when the pipe is advanced in the guide cylinder 56 and the bending die 57, a bending moment is generated on the pipe 55 between the guide cylinder 56 and the bending die 57. The distance V between the guide cylinder 56 and the bending die 57, and the offset u between the bending die 57 and the central axis line of the guide cylinder 56, are both changeable so that bent pipes having various radii can be produced.
In the prior art described above, before starting to bend the pipe, the bending die 57 is located an the central line of the guide cylinder 56 (offset u=0). Upon advancing the pipe 55 in the direction E in FIG. 5, the pipe 55 enters the bending die 57 which moves to deviate from the central axis line of the guide cylinder 56 (offset u&gt;0). By further advancing the pipe 55 in this condition, the pipe is bent with a predetermined radius.
An end portion of the pipe 55 passed through the bending die 57 before the bending die 57 arrives at the offset u is not formed with the predetermined radius, but with a radius larger than the predetermined radius. This is because sufficient bending force by the bending moment caused by the offset u is not added to this end portion of the pipe 55.
In the two methods of forming the bent pipe explained above, it is not possible to deform the end portion of the pipe to a predetermined radius. Thus, in order to produce a bent pipe having a predetermined radius, it is necessary to cut off the end portion of the pipe.